An onion is a round vegetable that grows underground, made up of many thin layers wrapped tightly around each other. When you slice an onion, you can peel these layers apart like the pages of a book, each one curved and papery thin.

Onions have a sharp, distinctive smell and taste that mellows and sweetens when you cook them. They're a foundation ingredient in cooking across nearly every culture: French chefs start sauces with onions, Mexican cooks use them in salsa, and Indian recipes often begin by frying onions until golden. Raw onions taste strong and pungent, but cooked slowly in a pan, they turn soft, golden, and surprisingly sweet.

The onion's infamous reputation comes from what happens when you cut one: it releases invisible chemicals into the air that sting your eyes and make them water. This is the onion's natural defense against animals that might want to eat it. Some cooks chill their onions first or cut them near running water to reduce the tears.

The word can also describe something with many layers. When someone describes a mystery story as having “layers like an onion,” they mean you keep discovering new depths the more you explore it.